(PDK-997) Remove Dir.chdir call from check:test_file task

This `chdir` breaks the isolation of the rake task and affects the
behaviour of other tasks that are invoked in it in the same `rake`
execution.

Rather than modify the task so that it changes back to the parent
directory after the task has finished, we can just remove the `chdir`
entirely and just change the glob to look for files in the directory.

This PR also adds some basic unit tests for the `check:test_file` task.

lib/puppetlabs_spec_helper/rake_tasks.rb

@@ -293,13 +293,10 @@
 
   desc 'Fails if .pp files present in tests folder'
   task :test_file do
-    if Dir.exist?('tests')
-      Dir.chdir('tests')
-      ppfiles = Dir['*.pp']
-      unless ppfiles.empty?
-        puts ppfiles
-        raise '.pp files present in tests folder; Move them to an examples folder following the new convention'
-      end
+    ppfiles = Dir[File.join('tests', '**', '*.pp')]
+    unless ppfiles.empty?
+      puts ppfiles
+      raise '.pp files present in tests folder; Move them to an examples folder following the new convention'
     end
   end
 

spec/spec_helper.rb

@@ -25,10 +25,18 @@
 require 'puppetlabs_spec_helper/puppetlabs_spec/puppet_internals'
 require 'puppetlabs_spec_helper/rake_tasks'
 
+RSpec.shared_context 'rake task', type: :task do
+  subject(:task) { Rake::Task[task_name] }
+
+  let(:task_name) { self.class.top_level_description.sub(%r{\Arake }, '') }
+end
+
 # configure RSpec after including all the code
 RSpec.configure do |config|
   config.expect_with :rspec do |c|
     c.syntax = :expect
   end
   config.mock_with :rspec
+
+  config.include_context 'rake task', type: :task
 end

spec/unit/puppetlabs_spec_helper/tasks/check_test_file_spec.rb

@@ -0,0 +1,28 @@
+require 'spec_helper'
+
+describe 'rake check:test_file', type: :task do
+  before(:each) do
+    test_pattern = File.join('tests', '**', '*.pp')
+    allow(Dir).to receive(:[]).with(test_pattern).and_return(test_files)
+  end
+
+  context 'when there are .pp files under tasks/' do
+    let(:test_files) { [File.join('tests', 'an_example.pp')] }
+
+    it 'raises an error' do
+      expected_output = test_files.join("\n")
+
+      expect { task.execute }
+        .to raise_error(%r{pp files present in tests folder})
+        .and output(a_string_including(expected_output)).to_stdout
+    end
+  end
+
+  context 'when there are no .pp files under tasks/' do
+    let(:test_files) { [] }
+
+    it 'runs without raising an error' do
+      expect { task.execute }.not_to raise_error
+    end
+  end
+end